Recently, a light emitting device using a light emitting element including a light emitting diode (LED) has been used as a backlight of a liquid crystal panel or a lighting system. FIG. 1 is a circuit diagram illustrating a configuration example of a light emitting device according to a comparison technique. A light emitting device 1003 includes a plurality of LED strings 1006_1˜1006—n, a switching power source 1004, and a current driving circuit 1008.
Each of the LED strings 1006 includes a plurality of LEDs connected in series. The switching power source 1004 boosts an input voltage Vin and supplies a driving voltage Vout to one end portion of the LED strings 1006_1˜1006—n. 
The current driving circuit 1008 includes current sources CS1˜CSn installed at the respective LED strings 1006_1˜1006—n. The respective current sources CS supply a driving current ILED, which is based on target luminance, to the corresponding LED strings 1006.
The switching power source 1004 includes an output circuit 1102 and a control IC 1100. The output circuit 1102 includes an inductor L1, a switching transistor M1, a rectifying diode D1, and an output capacitor C1. The control IC 1100 feedback-controls a duty ratio of ON/OFF operations of the switching transistor M1 such that the lowest one among voltages VLED1˜VLEDn (also called detection voltages) generated from each of cathode terminals of the LED strings 1006_1˜1006—n is close to a target voltage Vref. As a result, an output voltage Vout from the switching power source 1004 is stabilized to (Vref+Vf). In this configuration, Vf indicates a forward voltage (voltage drop) of the LED strings 1006.
In such a light emitting device 1003, to adjust the luminance of the LED strings 1006, the driving current ILED is often pulse width modulation (PWM)-controlled. More specifically, a PWM controller 1009 of the current driving circuit 1008 generates burst dimming pulses PWM1˜PWMn, each having a duty ratio based on luminance, and controls switching of the current sources CS1˜CSn that correspond to the burst dimming pulses PWM1˜PWMn, respectively. Such controlling is also referred to as burst dimming or burst controlling.
Such a light emitting device is generally known to have the following problems.
During a period in which the current source CS is in an OFF state, namely, during a turn-off period of the LED strings 1006, the detection voltage VLED is negated, so it is difficult to perform feedback controlling based on the detection voltage VLED. Thus, the control IC 1100 adjusts the duty ratio of ON/OFF operations of the switching transistor M1 based on the detection voltage VLED during a period in which the current source CS is in an ON state, namely, during a turn-on period of the LED strings 1006.
Further, when the turn-on period of the LED strings 1006 is shortened, the period during which feedback controlling is valid is shortened. When the turn-on period becomes as short as a switching pulse of the switching transistor M1 of the switching power source, feedback by an error amplifier cannot be followed, degrading the driving voltage Vout. Therefore, during the turn-on period, the luminance of the LED strings 1006 is degraded or the LED strings 1006 may not emit light.
The applicant of the present disclosure notes that the above problems are not considered common general knowledge in the field of the present disclosure. In other words, the foregoing discussion was first made by the applicant of the present disclosure.